The present invention is directed, in general, to polishing of semiconductor wafers and, more specifically, to a system and method of determining a polishing endpoint by monitoring signal intensity during a polishing process.
In the fabrication of semiconductor components, metal conductor lines are formed over a substrate containing device circuitry. The metal conductor lines serve to interconnect discrete devices, and thus form integrated circuits (ICs). The metal conductor lines are further insulated from the next interconnection level by thin films of insulating material deposited by, for example, Chemical Vapor Deposition (CVD) of oxide or application of Spin On Glass (SOG) layers followed by fellow processes. Holes, or vias, formed through the insulating layers provide electrical connectivity between successive conductive interconnection layers. In such wiring processes, it is desirable that the insulating layers have a smooth surface topography, since it is difficult to lithographically image and pattern layers applied to rough surfaces.
Also, deep (greater than 3 xcexcm) and narrow (less than 2 xcexcm) trench structures have been used in advanced semiconductor design for three major purposes: (1) to prevent latch-up and to isolate n-channel from p-channel devices in CMOS circuits; (2) to isolate the transistors of bipolar circuits; and (3) to serve as storage-capacitor structures in DRAMS. However, in this technology it is even more crucial to precisely determine the endpoint of differing materials to prevent unnecessary dishing out of the connector metal.
Chemical-mechanical polishing (CMP) has been developed for providing smooth insulator topographies. Briefly, the CMP processes involve holding and rotating a thin, reasonably flat semiconductor wafer against a wetted polishing surface under controlled chemical, pressure, and temperature conditions. A chemical slurry containing a polishing agent, such as alumina or silica, is used as the abrasive material. Additionally, the chemical slurry contains selected chemicals that etch or oxidize various surfaces of the wafer during processing. The combination of mechanical and chemical removal of material during polishing results in superior planarity of the polished surface.
CMP is also used to remove different layers of material from the surface of a semiconductor wafer. For example, following via formation in a dielectric material layer, a metallization layer is blanket-deposited, and then CMP is used to produce planar metal studs. When used for this purpose, it is important to remove a sufficient amount of material to provide a smooth surface, without removing an excessive amount of underlying materials. The accurate removal of material is particularly important in today""s submicron technologies where the layers between device and metal levels are constantly getting thinner. To better determine endpoints between removed and remaining layers of a semiconductor wafer, an accurate polishing endpoint detection technique is invaluable.
In the past, endpoints have been detected by interrupting the CMP process, removing the wafer from the polishing apparatus, and physically examining the wafer surface by techniques that ascertain film thickness and/or surface topography. However, with such prior art processes if the wafer did not meet specifications, it was loaded back into the polishing apparatus for further polishing to achieve the desired planarity. This would have to be repeated until a sufficient amount of material was removed. Unfortunately, in addition to the excess time required by this technique, if too much material was removed, the wafer was likely found to be substandard to the required specifications, and often discarded altogether. By experience, an elapsed CMP time for a given CMP process has been developed with some accuracy. However, like the prior art technique just mentioned, this endpoint detection technique is time consuming, unreliable, and costly.
Various active processes have been developed to circumvent the problems associated with prior art endpoint detection techniques. However, these active processes suffer from their own disadvantages and inaccuracies. One of the better known of these prior art techniques involves the continuous monitoring of the motor current of the CMP apparatus. Specifically, the drive motor used to rotate the platen holding the polishing pad is continuously monitored during the polishing process for changes in load current. As each layer of a semiconductor wafer is polished, a certain amount of friction develops between the polishing pad and differing wafer layers. When the CMP process finishes the removal of one layer of the wafer and begins on the next, a change in the amount of friction between the polishing pad and wafer layer affects the amount of work required by the drive motor. As the work required by the drive motor changes with each different layer, the load current of the motor changes as well. These changes in load current may be monitored to determine when the polishing process has begun on a new wafer layer.
Unfortunately, this technique is typically successful for detecting the endpoint of only metal layers, and has proven inaccurate for use with dielectric and other non-metal layers. Other factors, including the various slurries that may be used depending on the desired result, may affect the current of the drive motor, leading to inaccurate results. Also, changes in load current caused by a power surge may incorrectly inform the operator that an endpoint of a particular layer of the wafer has been reached.
Another common technique found in the prior art is optical endpoint detection. In this technique, a laser, mounted in the platen, is transmitted through a window in the polishing pad and contacts the layer on the wafer currently being polished. A change in layer material may be detected by the laser to determine an endpoint of a particular layer. However, this technique may also be deficient in that problems with the window in the polishing pad can lead to inaccurate results. For instance, leakage of slurry, or even water, onto the window may distort the laser beam and detrimentally affect detection. Also, damage to the window, perhaps from a manufacturing defect or even caused by an operator mounting the polishing pad, may also prevent or alter endpoint detection. Even if the window is not affected, those skilled in the art understand the excess cost associated with such specialized polishing pads.
Still other techniques for endpoint detection found in the prior art include those techniques that bounce an acoustic signal off of the wafer layers being polished, similar to sonar principles. However, these prior art detection techniques are based on the time (or speed) of a round trip of the acoustic waves directed to, and reflected back from, the wafer layers. Unfortunately, if such techniques were employed during a polishing operation, when endpoint detection would be most beneficial, excess layer thickness may be removed while waiting to measure the time of a return trip of the waves from the layer. Such a deficiency may become even more critical when only a small thickness, for example, a few microns, needs to be polished from the wafer 120. Those skilled in the art understand that over-polishing a wafer layer by just a few microns may render dies in the wafer, or perhaps the entire wafer, unusable. With the high costs of semiconductor materials in today""s competitive semiconductor market, manufacturers are understandably eager to avoid wasting product.
Thus, a more reliable and accurate technique for determining a polishing endpoint, with less risk than those found in the prior art, is desirable. Accordingly, what is needed in the art is an improved technique for accurately determining the endpoint of one semiconductor wafer layer and the beginning of the next during a polishing process that does not suffer from the deficiencies of the techniques found in the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a polishing endpoint detection system, for use with a polishing apparatus. In one embodiment, the polishing endpoint detection system includes a carrier head having a polishing platen associated therewith. Also, the detection system includes a signal emitter located adjacent one of the carrier head or polishing platen. The signal emitter is configured to generate an emitted signal capable of traveling through an object to be polished. In addition, the detection system includes a signal receiver located adjacent another of the carrier head or polishing platen. The signal receiver is configured to receive the emitted signal from which a change in a signal intensity of the emitted signal can be determined.
In another aspect, the present invention provides a method of determining a polishing endpoint of a surface located on a semiconductor wafer. In one embodiment, the method includes emitting a first signal from an emitter located adjacent one of a carrier head or a polishing platen. The method further includes causing the signal to pass through a polished film located on a semiconductor wafer, and thereby provide a second signal having a signal intensity less than a signal intensity of the first signal. The method also includes receiving the second signal emanating from the film with a receiver located adjacent another of the carrier head or the polishing platen. The method then includes determining a polishing endpoint for the film as a function of a change of intensity between the first and second signals.
In yet another aspect, the present invention provides a method of manufacturing an integrated circuit. In one embodiment, the method includes forming an integrated circuit layer on a semiconductor wafer. The integrated circuit layer is polished with a polishing apparatus having a carrier head and a polishing platen associated therewith. The method further includes determining a polishing endpoint of the integrated circuit layer by emitting a first signal from an emitter located adjacent one of the carrier head or the polishing platen and causing the first signal to pass through the integrated circuit layer. A second signal is thereby provided having a signal intensity less than a signal intensity of the first signal. The method includes receiving the second signal emanating from the integrated circuit layer with a receiver located adjacent another of the carrier head or the polishing platen. The method still further includes determining the polishing endpoint as a function of a change of intensity between the first and second signals.
The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.